This invention relates to a circuit arrangement for feeding a single-phase or multi-phase load from an energy supply mains which carries an at least substantially sine-wave mains alternating voltage.
When electronic energy converters are fed from an energy supply mains constructed as a single-phase alternating current mains, a full-wave bridge rectifier circuit with a subsequent storage capacitor are often used at the mains side. This bridge rectifier circuit, also referred to as a Graetz bridge, ensures that a mains current for recharging the storage capacitor flows from the energy supply mains only if when the instantaneous value of the voltage in the energy supply mains is greater than the voltage across the storage capacitor. A pronounced pulse-like current is thus drawn from the energy supply mains, the mean value of said current corresponding to the mean value of the current drawn from the storage capacitor by a load. The mains current thus formed, drawn from the energy supply mains, has a high harmonic content. If such a Graetz bridge is used to operate a high-power load, the harmonic content will very quickly exceed a limit specified as the highest permissible harmonic content by the operators of the energy supply mains. Loads generating a mains current with a higher harmonic content are not permissible, considering the directives imposed by the operators of energy supply mains and also considering the prevailing national and European standards.
It is known to connect a filter choke downstream from a Graetz bridge; such a choke achieves smoothing of the mains current and hence a reduction of the harmonic content. However, such a filter choke becomes very voluminous and heavy in the case of high powers. This has an adverse effect on the compactness and weight of apparatus equipped with such power supply units.
The publication "Schaltnetzteile" by W. Hirschmann and A. Hauenstein, published by Siemens, ISBN No. 3-8009-1550-2, section 6.4, pp. 441 to 444, describes an up-converter which draws a current. This arrangement, also referred to as a preconditioner, includes an electronic power switch, a high-frequency choke and a very fast switching diode. The power switch must be driven by an appropriate control circuit which controls the mains current in a sine-wave fashion without causing a significant fluctuation of the voltage in the intermediate circuit, i.e. downstream from the Graetz bridge. When the control circuit is suitably conceived, the cited circuit arrangement should enable a sine-wave mains current, the harmonics in the mains current should be at least substantially suppressable, irrespective of a load connected to this circuit arrangement, and it should also be possible to keep the power of the load constant in conformity with the at least approximately constant voltage in the intermediate circuit. A comparatively complex circuit means, however, is required for these purposes.
In many cases it is not necessary to feed a load with a power which is constant in time. It often suffices for the instantaneous value of this power to fluctuate about a mean value, for example, at double the mains frequency. The use of the above-mentioned preconditioner would be particularly disadvantageous in such cases.